1. Field of the Invention
The present invention relates to an organic electric field light-emitting element (may also referred to as an “organic electroluminescence element”, an “organic EL element”, and the like).
2. Description of the Related Art
Organic electric field light-emitting elements have characteristics such as of self luminescence and high-speed response, and hence are expected as applications for flat panel displays. Especially since a two layer (laminate) type of organic electric field light-emitting elements, in which a thin hole-transporting organic film (i.e. a hole transporting layer) and a thin electron-transporting organic film (i.e. an electron transporting layer) are laminated were reported, organic electric field light-emitting elements attracted great deals of interests as large-scale luminescence elements which emits light with low voltage of 10 V or lower. The basic structure of the laminate type of the organic electric field light-emitting element is a laminate structure of a positive electrode, a hole transporting layer, a light emitting layer, an electron transporting layer, and a negative electrode.
Such organic electric field light-emitting element loses its carrier balance when the current density changes, which causes decreased emission efficiency or permanence, and chromaticity changes of emission. Among these problems, the main factor for causing the chromaticity change is an influence from optical interference due to the change of the location where light is emitted. Due to the optical interference, the color of the emission is tinted with blue as the location is closer to the electrode, and is tinted with red as the location is distant from the electrode. This phenomenon is more significantly shown as the light-emitting layer is thicker.
To solve this problem, for example, in Japanese Patent Application Laid-Open (JP-A) No. 2009-55010, the following organic electric field light-emitting element is proposed. This organic electric field light-emitting element contains an organic layer containing a light-emitting layer, disposed between a pair of electrodes. This light-emitting layer contains at least two luminescent materials each having a different value of electron affinity (Ea) or ionization potential (Ip), and at least one host material, where the concentration of the luminescent material having the larger value of Ea or Ip among the two luminescent materials gradually decreases in the light-emitting layer in the direction from the cathode side to the anode side of the light-emitting layer, and the concentration of the luminescent material having the smaller value of Ea or Ip gradually decreases in the direction from the anode side to the cathode side of the light-emitting layer.
Moreover, proposed in JP-A No. 2001-189193 is a light-emitting element having a light-emitting region between an anode and a cathode, in which the light-emitting region is consisted of a material contributing to emission and a medium to include such material, and the light-emitting region has an almost continuously changing concentration distribution of the material contributing to emission in the direction from the anode side to the cathode side of the light-emitting region.
However, the control of the luminescent material concentration disclosed in the patent literatures is aimed to improve emission efficiency and permanence by controlling a carrier balance, and no consideration is paid for preventing chromaticity change caused by current density.
Accordingly, it has been desired to promptly provide an organic electric field light-emitting element, which has improved emission efficiency and permanence as well as capable of preventing a chromaticity change when current density is changed, by changing the concentration of a doping material to prevent the color sift caused by optical interference.